I've tested once again with Firefox 62.0.3 and it worked for me.
You can monitor the log produced by Fox Contact during the next few days in order to ensure that it does not happen to your users as well.
Just download and open the file administrator/logs/foxcontact.php or /logs/foxcontact.php depending on your settings.
In case someone is blocked you'll see an entry like this
Spam attempt blocked by module [...]
followed by a dump of the form data, including user's browser and operating system